#include "blaswrap.h"
#include "f2c.h"

/* Subroutine */ int slasd5_(integer *i__, real *d__, real *z__, real *delta, 
	real *rho, real *dsigma, real *work)
{
/*  -- LAPACK auxiliary routine (version 3.0) --   
       Univ. of Tennessee, Oak Ridge National Lab, Argonne National Lab,   
       Courant Institute, NAG Ltd., and Rice University   
       June 30, 1999   


    Purpose   
    =======   

    This subroutine computes the square root of the I-th eigenvalue   
    of a positive symmetric rank-one modification of a 2-by-2 diagonal   
    matrix   

               diag( D ) * diag( D ) +  RHO *  Z * transpose(Z) .   

    The diagonal entries in the array D are assumed to satisfy   

               0 <= D(i) < D(j)  for  i < j .   

    We also assume RHO > 0 and that the Euclidean norm of the vector   
    Z is one.   

    Arguments   
    =========   

    I      (input) INTEGER   
           The index of the eigenvalue to be computed.  I = 1 or I = 2.   

    D      (input) REAL array, dimension ( 2 )   
           The original eigenvalues.  We assume 0 <= D(1) < D(2).   

    Z      (input) REAL array, dimension ( 2 )   
           The components of the updating vector.   

    DELTA  (output) REAL array, dimension ( 2 )   
           Contains (D(j) - lambda_I) in its  j-th component.   
           The vector DELTA contains the information necessary   
           to construct the eigenvectors.   

    RHO    (input) REAL   
           The scalar in the symmetric updating formula.   

    DSIGMA (output) REAL   
           The computed lambda_I, the I-th updated eigenvalue.   

    WORK   (workspace) REAL array, dimension ( 2 )   
           WORK contains (D(j) + sigma_I) in its  j-th component.   

    Further Details   
    ===============   

    Based on contributions by   
       Ren-Cang Li, Computer Science Division, University of California   
       at Berkeley, USA   

    =====================================================================   


       Parameter adjustments */
    /* System generated locals */
    real r__1;
    /* Builtin functions */
    double sqrt(doublereal);
    /* Local variables */
    static real b, c__, w, delsq, del, tau;

    --work;
    --delta;
    --z__;
    --d__;

    /* Function Body */
    del = d__[2] - d__[1];
    delsq = del * (d__[2] + d__[1]);
    if (*i__ == 1) {
	w = *rho * 4.f * (z__[2] * z__[2] / (d__[1] + d__[2] * 3.f) - z__[1] *
		 z__[1] / (d__[1] * 3.f + d__[2])) / del + 1.f;
	if (w > 0.f) {
	    b = delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	    c__ = *rho * z__[1] * z__[1] * delsq;

/*           B > ZERO, always   

             The following TAU is DSIGMA * DSIGMA - D( 1 ) * D( 1 ) */

	    tau = c__ * 2.f / (b + sqrt((r__1 = b * b - c__ * 4.f, dabs(r__1))
		    ));

/*           The following TAU is DSIGMA - D( 1 ) */

	    tau /= d__[1] + sqrt(d__[1] * d__[1] + tau);
	    *dsigma = d__[1] + tau;
	    delta[1] = -tau;
	    delta[2] = del - tau;
	    work[1] = d__[1] * 2.f + tau;
	    work[2] = d__[1] + tau + d__[2];
/*           DELTA( 1 ) = -Z( 1 ) / TAU   
             DELTA( 2 ) = Z( 2 ) / ( DEL-TAU ) */
	} else {
	    b = -delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	    c__ = *rho * z__[2] * z__[2] * delsq;

/*           The following TAU is DSIGMA * DSIGMA - D( 2 ) * D( 2 ) */

	    if (b > 0.f) {
		tau = c__ * -2.f / (b + sqrt(b * b + c__ * 4.f));
	    } else {
		tau = (b - sqrt(b * b + c__ * 4.f)) / 2.f;
	    }

/*           The following TAU is DSIGMA - D( 2 ) */

	    tau /= d__[2] + sqrt((r__1 = d__[2] * d__[2] + tau, dabs(r__1)));
	    *dsigma = d__[2] + tau;
	    delta[1] = -(del + tau);
	    delta[2] = -tau;
	    work[1] = d__[1] + tau + d__[2];
	    work[2] = d__[2] * 2.f + tau;
/*           DELTA( 1 ) = -Z( 1 ) / ( DEL+TAU )   
             DELTA( 2 ) = -Z( 2 ) / TAU */
	}
/*        TEMP = SQRT( DELTA( 1 )*DELTA( 1 )+DELTA( 2 )*DELTA( 2 ) )   
          DELTA( 1 ) = DELTA( 1 ) / TEMP   
          DELTA( 2 ) = DELTA( 2 ) / TEMP */
    } else {

/*        Now I=2 */

	b = -delsq + *rho * (z__[1] * z__[1] + z__[2] * z__[2]);
	c__ = *rho * z__[2] * z__[2] * delsq;

/*        The following TAU is DSIGMA * DSIGMA - D( 2 ) * D( 2 ) */

	if (b > 0.f) {
	    tau = (b + sqrt(b * b + c__ * 4.f)) / 2.f;
	} else {
	    tau = c__ * 2.f / (-b + sqrt(b * b + c__ * 4.f));
	}

/*        The following TAU is DSIGMA - D( 2 ) */

	tau /= d__[2] + sqrt(d__[2] * d__[2] + tau);
	*dsigma = d__[2] + tau;
	delta[1] = -(del + tau);
	delta[2] = -tau;
	work[1] = d__[1] + tau + d__[2];
	work[2] = d__[2] * 2.f + tau;
/*        DELTA( 1 ) = -Z( 1 ) / ( DEL+TAU )   
          DELTA( 2 ) = -Z( 2 ) / TAU   
          TEMP = SQRT( DELTA( 1 )*DELTA( 1 )+DELTA( 2 )*DELTA( 2 ) )   
          DELTA( 1 ) = DELTA( 1 ) / TEMP   
          DELTA( 2 ) = DELTA( 2 ) / TEMP */
    }
    return 0;

/*     End of SLASD5 */

} /* slasd5_ */

